The AR6 battery triode is a VR22 made for the British Army. It was a popular type and had many equivalents. The shape of the etching on the envelope suggests that this exhibit was made by M-OV.
As an audio output valve it produced 0.18 Watts.
Power was a wet accumulator for the filament and a dry battery for the anode supply.
The bright anode seen end on.
Some other lettering including the letters WD for War Department.
The filament held by tension springs and the grid wire.
The balloon envelope is 41 mm in diameter and, excluding the B4 base pins, is 85 mm tall.